IMAP and POP3 are both protocols used by mail servers but they are different on how they handle and store your mails.
This protocol lets you download the email messages and will delete them from the mail server automatically. You can only access the emails from the device where you have downloaded them. This protocol doesn’t need a constant connection while emails are being accessed and it can help conserve disk space utilization.
On this protocol, the email messages are stored permanently in the mail server. You may access your emails from any device you log in to using your email account credentials. This protocol needs more dedicated disk space and constant connection while accessing and reading the emails.
NOTE: These protocols can be enabled and disabled in WHM. (WHM > Home > Service Configuration > Mailserver Configuration).